Gas turbine engines generally include a compressor for compressing in-coming air. The air is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases in turn flow to a turbine. The turbine extracts energy from the gases for driving a shaft. The shaft powers the compressor and generally another load such as an electrical generator.
Exhaust emissions from the combustion gases are a concern and are subject to mandated limits. Certain types of gas turbine engines are designed for low exhaust emissions operation, and in particular, for low NOx (nitrogen oxides) operation, minimal combustion dynamics, and ample auto-ignition and flame holding margins. Low NOx combustors are typically in a form of a number of burner cans circumferentially adjoining each other around the circumference of the engine. Each burner may have a swirler position therein. The swirlers may have a number of circumferentially spaced apart vanes for swirling and mixing the compressed air and fuel as they pass therethrough.
One issue with known gas turbine engines is the need to make the fuel/air mixture as homogenous as possible and the Wobbe index of the fuel/air mixtures as consistent as possible. In the past, the Wobbe index has been controlled with external fuel heating. The issues of the flame holding, auto-ignition margins, and the homogeneous mixing of the fuel and air have been addressed in part by changing the angles on the fuel nozzle swirl vanes and/or by changing the method by which the fuel is introduced to the air or vice versa, i.e., a cross-flow or coaxial flow may be used. The more homogeneous the flow, the more efficient the combustion process may be while producing fewer emissions.
There is a desire, therefore, for a gas turbine engine with improved fuel/air mixing, combustion dynamics, Wobbe control, and flame holding, auto-ignition margin, particularly in the context of low NOx combustion. The improved mixing should be accomplished without loss of engine efficiency.